The present invention relates to a process for forming a removable sealing connection between a tube and a sleeve, of the type comprising an olive shaped piece called "olive" in what follows, fast with the end portion of the tube and a nut having a threaded connection with the sleeve and comprising means for deforming the olive so as to apply it sealingly against an internal face of the sleeve when the nut is screwed on the sleeve.
This process is widely used at the present time for sealingly connecting metal ducts and particularly ducts of small diameter (a few millimeters to a few tens of millimeters) which have to withstand high internal pressures (British Pat. No. 1 365 815).
Two solutions are generally used for securing the olive to the tube. A first solution consists in crimping the olive on the tube. In this case an olive may more particularly be used made from elastomer material and crimped by means of a metal ring. The second solution consists in using an olive made from a metal slightly harder than that of the tube. In this case, when the olive is compressed by the nut during a first precrimping operation, it becomes embedded in the tube and subsequently remains interlocked therewith. The tube is ready to be connected, provided with a nut, a sleeve and the precrimped sealing olive. In that case, for example, an olive may be used made from brass when the tube is made from copper, an olive made fom treated steel when the tube is made for stainless steel. In the latter case, a grooving operation must be carried out before precrimping.
FIG. 1 shows a removable sealed connection using the second solution. The connection is to be made between the end portion of a tube 10 and a sleeve 12 having a shoulder facing the endmost edge of tube 10 and a threaded portion for receiving a nut 14. With the sealing olive 16 positioned by precrimping on the tube, nut 14 is slid along tube 10. A clearance is provided between the latter and the shoulder of sleeve 12. Nut 14 is then screwed tight. Nut 14 has an internal tapered surface 18 which comes into abutment with the olive and applies it against an internal frustoconical surface 20 of the sleeve. The pressure thus exerted on the olive deforms from the shape shown in dot-dash line to the shape shown with a continuous line in FIG. 1. Sealing is thus provided between olive 16 and tube 10 by an anchorage along line 22 and a curvilinear contact along line 23. It is provided, between olive 16 and sleeve 12, by abutting contact in the annular zone 24.
These known solutions however have different disadvantages. They do not guarantee a definitive and reliable crimping of the olive after numerous disassembly and reassembly operations. The sealing is only achieved along substantially linear contacts, which is unsatisfactory when the pressures considered are high. Before crimping the sealing olive in the case of a stainless steel tube, two operations must be carefully carried out using two specific tools: a grooving machine whose cutting wheel must be in perfect condition, and a pre-crimping unit. Furthermore, at least in the embodiment shown in FIG. 1, there is a concentration of forces which may cause minute cracks in the tube along line 22 and the creation of fatique points, to which pregrooving of the tube on which the olive is crimped further contributes. The tightening torque exerted must be exactly controlled, in order to provide sealing, by using a torque wrench.
It is an object of the invention to provide an improved process for forming a removable fluid tight connection of the above defined type. It is a more particular object to provide a process which overcomes the above disadvantages and guarantees long term reliability of the connection.
For that purpose, the olive is permanently secured to the tube by electromagnetic forming, whereby the connection is rendered fluid tight and the olive takes a reproducible and accurately defined shape.
Electromagnetic forming is a well known technique (British Pat. No. 2 086 284): it uses a burst of electromagnetic energy created by a brief high frequency discharge of high voltage electric energy in an inductive coil. It comprises two variants, known respectively under the name of "magneto forming" and "electroforming". In magneto forming, an electro-magnetic field propels the piece to be shaped, (which must be at least partially electrically conducting metal), at high speed against another piece forming a die whose shape it assumes. In electro forming (also known as electro-hydraulic forming), an electric pulse is applied to an explosive wire placed in an insulating and incompressible medium. The explosion creates a shock wave which is transmitted through the incompressible medium to the piece to be shaped so as to cause expansion thereof.
One or the other of the electro-magnetic forming processes will be selected depending on the diameter of the duct and the nature of the materials.
In the case of tubular ducts of small diameter (10 mm or so) and a few millimeters in thickness, crimping of the olive will be advantageously provided by shrinking using a magneto forming coil. This solution will be of particular interest for the pipes having an outer diameter of 12 mm and a thickness of two millimeters which are subjected to a pressure of from 200 to 400 bars in hydraulic control circuits.
When the diameter of the tube exceeds 20 mm, which is the general case in power transmission circuits, crimping may be obtained by expansion, using a magneto forming coil inserted in the tube. For smaller diameters, expansion crimping may be retained, but by electro forming using an explosive wire placed in the tube. Expansion crimping using a forming die has the advantage of providing, in a single operation crimping of the olive on the tube and suitable shaping thereof from a blank produced with wide tolerances. The forming die is more easily extracted than in the previous case. During such forming, there is no lengthening of the tube; the material is displaced in thickness and this deformation may be adjusted (FIG. 3). This technique further guarantees that the mechanical qualities of the stressed material (olive and tube) are retained and even provides surface hardening of the material by cold drawing.
The invention will be better understood from the following description of particular embodiments of the invention given by way of examples.